The goal of this project is to decipher the cellular signals that instigate leukocyte infiltration into the testis after exposure to mono-(2-ethylhexyl) phthalate (MEHP) and to determine the functional significance of these cells in the pathogenesis of MEHP-induced germ cell apoptosis. We have previously revealed a paracrine interaction between Sertoli cells (SCs) and germ cells (GCs) that initiates GCs to undergo apoptosis via FasL- Fas signaling. Central to this pathway is the inhibition of TIMP2 (tissue inhibitor of metalloproteinase 2) production by SCs after MEHP exposure. This allows for the consequent activation of matrix metalloproteinase 2 (MMP2) in the adluminal space that both generates a soluble form of tumor necrosis factor-? (sTNF?) as well as cause a disorganization of proteins that compose the blood-testis barrier (BTB) between adjacent SCs. A preliminary characterization of Fischer rat testis after MEHP (1 g/kg, p.o.) treatment at two peripubertal ages (PND 28 and 35) showed a robust infiltration of CD11b+ immunoreactive cells in the interstitial space prior to the peak induction of GC apoptosis. The CD11b antibody recognizes an antigen expressed on macrophages, neutrophils, monocytes, natural killer and dendritic cells. Intriguingly, we observed that the most pronounced infiltration of these cells occurs in peripubertal aged rats versus mature rats; and that C57BL/6J mice at both ages do not have a significant infiltration in response to MEHP exposure. The differential species and age- dependent sensitivity to MEHP-induced testicular injury is well recognized, although the mechanisms that account for these differences remain unresolved. Further, preliminary studies also show a dramatic increase in the expression of monocyte chemoattractant protein-1 (MCP-1) in peritubular myoid cells (PTMCs) in testis from PND 28 rats, but not in mature rats. MCP-1 is a prototypical chemokine for signaling the influx of leukocytes into tissues and is itself secreted in response to cytokines such as sTNF?, IL-1?, Il-1? and/or interferon-?. Taken together, our previous work and preliminary data, have led to the development of the hypothesis that MEHP-induced SC injury incites the production of cytokines, such as sTNF?, that trigger PTMCs to release chemokines that initiate the infiltration of leukocytes into the testis and act to enhance the extent of GC apoptosis. Moreover, a provocative underlying hypothesis of this proposal is that leukocytes account, in part, for the mechanism of the observed age- and species-dependent sensitivity to MEHP. To test these hypotheses, the first specific aim will use both acute and repeated MEHP treatments to characterize the leukocyte subtypes and timing of their influx into the testis following exposure. In the second aim, the chemokines and cytokines elicited, as well as their specific cellular source in the testis, will be examined. In the last ai, the functional significance of testis leukocyte infiltration in the pathogenesis of MEHP-induced injury will be directly tested. It is predicted that insights gained from this work will be useful or predicting susceptible individuals and preventing human reproductive health risks this class of chemicals.